Thawing of the permafrost around a well transporting hot fluids through a permafrost zone is prevented by a cyclic refrigeration operation in which the well is refrigerated by natural convection refrigeration operated only when the ambient conditions are suitable for the transfer of heat from the well...http://www.google.es/patents/US3880236?utm_source=gb-gplus-sharePatente US3880236 - Method and apparatus for transporting hot fluids through a well traversing a permafrost zone

Method and apparatus for transporting hot fluids through a well traversing a permafrost zoneUS 3880236 A

Resumen

Thawing of the permafrost around a well transporting hot fluids through a permafrost zone is prevented by a cyclic refrigeration operation in which the well is refrigerated by natural convection refrigeration operated only when the ambient conditions are suitable for the transfer of heat from the well to the atmosphere. The refrigeration system is a closed system comprised of one or more insulated conduits for transporting liquid refrigerant downwardly through the refrigerated zone, an annulus in the well for conducting the liquid refrigerant to the surface, a heat exchanger located at the surface to cool the refrigerant by heat exchange with ambient air, a refrigerant reservoir to maintain a supply of refrigerant in the system, and a temperature responsive shutoff device to stop the circulation of refrigerant during periods of high ambient temperature. The difference in density of the liquid refrigerant in the insulated conduit and the annulus induces circulation of refrigerant through the system.

( Apr. 29, 1975 Prinmry E.\'aminer-Marion Parsons, Jr. Assistant E.ruminer-Lawrence .l. Staab Attorney. Agent, or Firm-Dean Sandford; Richard C. Hartman [57] ABSTRACT Thawing of the permafrost around a well transporting hot fluids through a permafrost zone is prevented by a cyclic refrigeration operation in which the well is refrigerated by natural convection refrigeration operated only when the ambient conditions are suitable for the transfer of heat from the well to the atmosphere. The refrigeration system is a closed system comprised of one or more insulated conduits for transporting liquid refrigerant downwardly through the refrigerated zone, an annulus in the well for conducting the liquid refrigerant to the surface, a heat exchanger located at the surface to cool the refrigerant by heat exchange with ambient air. a refrigerant reservoir to maintain a supply of refrigerant in the system. and a temperature responsive shutoff device to stop the circulation of refrigerant during periods of high ambient temperature.

F '1 'I 'T 'I 'l" T T 'I T OPERATING crcus l I I I l JAN I JAN r JA'N I JAN 1 JAN rmsr ram SECOND rue THIRD YEAR FOURTH YEAR FIFTH ram Fig 4 METHOD AND APPARATUS FOR TRANSPORTING HOT FLUIDS THROUGH A WELL TRAVERSING A PERMAFROST ZONE This invention relates to wells for recovering earth fluids, and more particularly to wells for transporting hot fluids through a permafrost zone.

Permafrost is perennially frozen ground found in the Arctic regions. The permafrost zone contains some layers of gravel free of ice known as dry permafrost. but the bulk of the zone is composed of rocks or of unconsolidated aggregates of sand, silt and gravel in which the interstitial water is frozen to ice. Permafrost is formed by the spreading downward of the cold found at the surface during the Arctic winters, particularly in regions of low snow fall. The downward spread of freezing temperatures to form permafrost continues until an equilibrium is reached with the heat flow from the earths interior. There is seasonal thawing and freezing at the surface, but thawing rarely penetrates more than 18 inches where the permafrost is protected by tundra or vegetation. Thus, the permafrost zone consists of frozen earth, the bulk of which remains permanently frozen unless some external source of heat is introduced that alters the temperature equilibrium. The thickness of the permafrost zone varies with latitude and with the particular geographic location.

Permafrost generally has sufficient strength to support oil exploration and recovery operations, so long as it remains frozenv However, when melted, unconsolidated permafrost shrinks and subsides causing a downdrag force on well casings passing through the permafrost zone. Drilling operations can generally be con ducted without causing any substantial melting of the permafrost. However, melting of the permafrost is experienced around wells conducting hot fluids through the permafrost zones, and this melting must be prevented or retarded sufficiently that subsidence does not occur during the life of the well.

Heat loss from a hot fluid transported through the permafrost can be substantially reduced by insulating the conduit carrying the hot fluid. In some cases, insulation alone reduces heat loss to the permafrost sufficiently to avoid melting the permafrost. However, insulation alone is generally not sufficient where the heat loss is relatively high or where the permafrost is initially relatively warm, i.e., where the permafrost is at a temperature only slightly below the freezing point of water. Various mechanical means such as slip joint casing and the like, which would not be damaged by subsidence, have been proposed for use in transporting hot fluids through permafrost. However, such devices are expensive to construct and install, and are not yet reliable. Mechanical refrigeration. usually in conjunction with insulation of the conduit carrying the hot fluid. can be employed to effectively prevent heat loss from the well to the surrounding permafrost. Although various methods of refrigerating the well have been proposed which are effective in preventing melting of the permafrost, mechanical refrigeration equipment is expensive to install and operate, requires a source of power at the well site, and requires at least the periodic attention of an operator. Thus, need exists for a simpler and less costly method for refrigerating wells traversing a permafrost zone.

Accordingly, it is a principal object of this invention to provide a simple, low cost method for conducting hot fluids through a well traversing a permafrost zone that substantially prevents thawing of the permafrost around the well.

Another object of the invention is to provide a method for preventing thawing of the permafrost adjacent to a well transporting hot fluid through a permafrost zone.

Still another object of the invention is to provide a method for producing hot oil from a reservoir underlying a permafrost zone that prevents thawing ofthe permafrost surrounding the well.

A further object of the invention is to provide a method for injecting hot fluids into an earth formation underlying a permafrost zone without thawing the permafrost surrounding the injection well.

A still further object of the invention is to provide an improved method for operating a refrigerated well conducting hot fluids through a permafrost zone.

An even further object is to provide a refrigerated well for transporting hot fluids through a permafrost zone.

Other objects and advantages of the invention will be apparent from the following description.

In brief, this invention concerns a method and apparatus for preventing thawing of the permafrost around a well transporting hot fluids through a permafrost zone. Thawing is prevented by a cyclic refrigeration operation in which the permafrost is subcooled by a natural convection refrigeration operation that transfers heat from the well and the surrounding permafrost to the atmosphere. The well is refrigerated only during those periods that the ambient air temperature is sufficiently low to permit the transfer of heat to the atmosphere. During periods that the refrigeration system is inoperative, the subcooled permafrost warms, but does not thaw to any appreciable extent.

The refrigeration system of this invention is a closed system comprised of one or more insulated conduits for transporting liquid refrigerant downwardly through the refrigerated zone, an annulus in the well for returning the liquid refrigerant to the surface in heat exchange relationship with the well fluids and the permafrost, a heat exchanger located at the surface to cool the refrigerant by heat exchange with ambient air, a refrigerant reservoir to maintain a supply of refrigerant in the system, and a temperature responsive shutoff device to stop the circulation of refrigerant during periods of high ambient air temperature. The difference in density of the liquid refrigerant in the insulated conduit and the annulus induces circulation of refrigerant through the system. Sufficient pressure is maintained on the system to prevent vaporization of the refrigerant.

The invention will be more readily understood by reference to the accompanying drawings, in which:

FIG. I is a vertical cross-sectional view schematically illustrating a well equipped with the refrigeration system of this invention traversing a permafrost zone;

FIG. 2 is a horizontal cross-sectional view of the well illustrated in FIG. I taken along the line 22 of FIG.

FIG. 3 is a graph illustrating the mean monthly temperature variation for a typical Arctic region; and

FIG. 4 is a graph illustrating the variation in the temperature of the permafrost at various distances from a typical well provided with the refrigeration system of this invention during the initial five year cycle.

Referring specifically to FIGS. 1 and 2, there is illustrated a well I0 completed in an earth formation comprised of an upper permanently frozen permafrost zone 12 and a lower unfrozen earth strata 14 that overlies a productive petroleum reservoir. not shown. Permafrost zone 12 is permanently frozen from surface 16 to a depth of several hundred feet, or more, with only the top few feet of the permafrost zone being normally subjected to any appreciable thawing during annual seasons of warm weather. Well is comprised of surface conductor 20 cemented in the upper strata of permafrost zone 12 with cement 22 that forms a sheath surrounding the exterior of conductor 20. Typically, surface conductor 20 is comprised of a length of relatively large diameter pipe, such as 24 or 30-inch diameter casing, 20 to 60 feet in length, and preferably about 40 feet in length.

Casing 24 is the second casing string run and preferably extends a substantial distance into the permafrost zone, i.e., to a depth of about 200 to about 700 feet and can extend completely through the permafrost zone. Although the diameter and weight of casing employed in this string will depend upon the targeted drilling depth. the types of formations traversed, and the total number of easing strings to be run; in a typical installation satisfactory for many wells, casing 24 is a 20-inch diameter casing cemented to the surface with cement 26 which forms a sheath around the exterior of casing 24 and fills the annulus between casing 24 and surface conductor 20.

Casing 30 is the permafrost string and extends from the surface to a depth below the permafrost zone and is cemented back to the bottom of casing 24 in conventional manner with cement 32 which forms a sheath surrounding casing 30. While the use of casing 30 is optional and may be omitted where the well can be drilled from the bottom of the permafrost through the producing interval without intermediate casing, in most instances it is preferred to employ a permafrost string terminating shortly below the bottom of the permafrost. Also, additional strings of intermediate casing may be installed where necessitated by conditions encountered in the drilling operation.

Casing 34 extends from the surface to the producing strata and is cemented back to the bottom of casing 30, or alternatively, if casing 30 is omitted, to the bottom of casing 24, with cement 36 which forms a sheath surrounding casing 34. The well is completed in conventional manner by extending casing 34 through the productive zones and perforating at selected intervals, or by terminating the casing above the productive zone and hanging a pre-slotted or pre-perforated liner in these zones. Production tubing 38 conveys produced fluids from the producing zone to the surface. or flow from the surface to the producing zones in the case of an injection well. A packer. or shoe, 40 can be provided on tubing string 38 to support insulation 42 in the annulus between production casing 34 and tubing 38. Insulation 42 can be any type of thermal insulation material such as polyurethane foam and the like. Insulation 42 extends the length of casing 24, and preferably extends from the surface to the bottom of permafrost zone 12.

The annulus between casing 30 and casing 34 can be filled with gelled diesel oil, or similar packing fluid, in conventional manner. The annulus between casing 24 and casing 30 is closed at the bottom by packer 50 to provide a closed refrigeration chamber 52 surrounding casing 30 and extending from the surface to a substantial depth in permafrost zone 12. At least one small diameter insulated tubing extending from the surface to the bottom of refrigeration chamber 52 is provided to permit a cooled liquid refrigerant to be introduced at the bottom of the refrigeration zone. In the illustrated embodiment, two tubes 54 and 56 extend to the bottom of refrigeration chamber 52. Tubes 54 and 56 are provided with insulation 58 and 60, respectively, extending substantially the length of the tubes. The maximum diameter the refrigeration tubes is determined by the radius of refrigeration chamber 52, and the number of tubes required is determined by the amount of refrigerant circulation desired and the desired flow velocity in these tubes.

The refrigeration system includes a suitable heat exchanger, such as finned air cooler 64, located at the surface and connected to refrigerant tubes 54 and 56 by conduit 62 and to the top of refrigeration chamber 52 by conduit 66 to complete the closed refrigeration circuit. Finned air cooler 64 can be provided with a motor driven fan, not shown, to force the circulation of air over the finned coils. However, preferably, finned air cooler 64 is provided with sufficient heat transfer surface to obtain the necessary cooling of the refrigerant with only natural air convection, thus avoiding the need for electric power to drive the fan and the increased maintenance requirement of the forced air system. A valve 68 responsive to temperature controller 70 which has a temperature measuring detector 72 exposed to the ambient air is provided in the refrigeration circuit to stop the circulation of refrigerant during those periods that the air temperature exceeds a predetermined value. In an optional embodiment, temperature controller 70 is provided with a second temperature sensor 74 buried in the permafrost at a depth below the seasonal thaw line. In this embodiment, temperature controller 70 actuates valve 68 to stop the circulation of refrigerant when the ambient air temperature warms up to the temperature of the permafrost as determined by temperature sensor 74, or approaches to a predetermined difference below the measured permafrost temperature.

A pressure tank connected to the refrigeration system by conduit 82 provides a reservoir or refrigerant so as to maintain the refrigerant system completely filled with refrigerant and to provide for thermal expansion and accommodate minor leakage from the system. Sufficient pressure is maintained on the refrigeration system to prevent vaporization of the refrigerant. Pressure can be conveniently maintained by introducing gas into pressure tank 80 from a gas supply source 84 at a pressure controlled by regulator 86 so as to maintain an atmosphere of gas in pressure tank 80 at the desired pressure. Where the gas is soluble in the refrigerant to any appreciable extent, it is preferred that a resilient membrane 88 be provided in pressure tank 80 to separate the gas from the liquid refrigerant. In this manner. a predetermined pressure is maintained on the refrigerant system and refrigerant can freely expand into pressure tank 80 or be displaced from the tank into the system to maintain the refrigerant system filled with liquid refrigerant.

A number of different materials can be employed as the refrigerant. Desirable refrigerant properties include:

I. A sufficiently high boiling point that the material remains in the liquid state over the entire operating temperature range.

2. Moderate thermal conductivity.

3. High thermal expansion.

4. Low viscosity.

5. Low thermal diffusity.

6. Low melting temperature.

Materials useful as refrigerants include low molecular weight liquid or liquifiable hydrocarbons such as propane, normal butane, iosbutane, pentane, and similar hydrocarbons containing up to about twelve carbon atoms per molecule, monoor poly-halogenated hydrocarbons containing one or more chlorine. bromine, iodine, or fluorine atoms per molecule, such as dichloromethane, difluoromethane, dichlorotetrafluoroethane, dichloromonofluoromethane, trichloromonofluoromethane. trichlorotrifluoroethane, dichloroethylene, trichloroethylene, and the like; low molecular weight monohydric and polyhydric alcohols; and aqueous brine solutions. Preferred materials useful as the refrigerant include propane. normal butane, isobutane, and mixtures of two or more of these materials.

The refrigeration system of this invention operates by natural convection. Cooled refrigerant exiting finned air cooler 64 passes through conduit 62 and downwardly through insulated refrigerant tubes 54 and 56, and is introduced into the bottom of annular refrigeration chamber 52. Because of the relatively high velocities in refrigerant tubes 54 and 56 and the thermal insulation provided these tubes, there is little heat conduction into the refrigerant and the refrigerant is introduced into the bottom of the refrigerant chamber at essentially the same temperature as it exits heat exchanger 64. Heat from the hot fluid in tubing 38 and from the permafrost warms the refrigerant in the annular refrigerant chamber, causing it to rise and causing the colder, more dense refrigerant in the refrigerant tubes to flow downwardly. Thus, the temperature difference between the refrigerant in refrigerant tubes 54 and 56 and in annular refrigeration chamber 52 produces a natural thermal syphon effect due to the difference in density of the refrigerant which causes refrigerant to circulate through the heat exchanger, down the refrigerant tubes and upwardly through the refrigeration chamber.

The thermal syphon effect is maximized by reducing the temperature of the refrigerant flowing downwardly through the refrigerant tubes. For this reason, refrigerant tubes 54 and 56 are insulated to minimize the amount of heat transferred to the downflowing refrigerant from the surrounding body of warmer refrigerant. and the size of the refrigerant tubes in relation to the cross-sectional flow area in surrounding refrigeration chamber 52 is selected so that the velocity of the downwflowing refrigerant is substantially higher than the velocity of the refrigerant flowing upwardly through the refrigeration chamber. The ratio of effective flow area of the refrigeration chamber to that of the refrigeration tubes should be from about 100:1 to about 4:1, and preferably from about :1 to about 10:1. With this configuration, refrigerant will flow downwardly through the refrigerant tubes at a velocity of about 0.2 to about l.0 foot per second. and upwardly through the refrigeration chamber at a velocity of 0.01 to 0.05 foot per second.

The annual temperature cycle produced by the refrigeration system of this invention is dependent in part upon the ambient air temperature at the surface. A typical monthly mean temperature for a representative Arctic region where permafrost is encountered, such as on the North Slope of Alaska, is illustrated in FIG. 3. The mean temperature shown there varies from below 20 F. during the winter period of December, January and February to above 32 F. during the summer period of June, July, August and September. It is apparent from the curve of monthly mean temperature illustrated in FIG. 3 that for an exemplary permafrost temperature of 28 F., the refrigeration system can be operated from the latter part of September through the latter part of May, whereas for a permafrost temperature of 20 F., the refrigeration system can be operated from the middle of October through the end of April. Of course, there are variations in these mean temperatures from year to year, and variations in the temperature from day to day and throughout the day which affect the operation of the refrigeration system.

When the refrigeration system is in operation, heat from the hot fluid flowing through tubing 38 transferred through insulation 42 and heat from the surrounding permafrost is removed. In this manner, the permafrost surrounding the well is subcooled during the winter operating period. When the ambient air temperature increases to the point that the refrigeration operation must be discontinued, heat losses from the well warm the subcooled permafrost, but not to the extent that the permafrost is melted. The result of this cyclic operation is illustrated in FIG. 4 which shows the calculated permafrost temperature at the casing and at distances of 5, I0 and 20 feet from the center line of the well during the initial five year period of operation of the refrigeration system. The abscissa of FIG. 4 indicates the operating cycle, i.e., the portion of each year that the refrigeration system is operated and that portion of the year that it is turned off. It can be seen that the temperature of the permafrost at the casing wall drops sharply from the initial permafrost temperature of 28 F. to a temperature approaching 20 F. during the first annual cycle, and that an annual temperature cycle is established during the first year of operation in which the temperature varies from about 20 F. to about 3 F. This temperature cycle is repeated so long as the operating conditions remained unchanged. FIG. 4 also shows that the annual temperature cycle is less pronounced at greater distances from the well, and that equilibrium temperature cycles are not reached as rapidly at these greater distances. FIG. 4 further shows that even though the refrigeration system is operated for a proportionately short period during the winter months, the permafrost surrounding the well is cooled to temperatures below the original permafrost temperature and that equilibrium temperature cycles are established at distances of up to 20 feet from the well within the first five years of operation.

Although the vapor-liquid refrigeration cycles are well known and have been proposed for use in refrigerating wells penetrating a permafrost zone, substantial problems are encountered with the two-phase vaporliquid or boiling system that are largely avoided with the single liquid phase system. By vapor-liquid or boiling system is meant a refrigerant system in which a liquid refrigerant is circulated to the annulus of the well,

is heated and vaporized in the annulus, and the vapors withdrawn for condensation and recirculation.

One major problem encountered in adapting the vapor-liquid refrigeration system to a long vertical column such as well refrigeration is that because of the hydrostatic head of the refrigerant in the annulus, the lower depths of the column of refrigerant will be at too high pressure to boil. and the refrigerant in these lower zones is heated to temperatures above that in the upper region of the annulus Most of the heat absorbed from the oil well and from the permafrost will occur in the boiling region near the top of the column. The heated liquid refrigerant will form convection loops within the annulus and the refrigerant in the lower section of the column will soon heat to a temperature above that of the permafrost. Thus, the transient currents ofwarmed refrigerant will begin to heat the permafrost rather than cool it. There will then be net cooling of the permafrost in the boiling zone at the top of the column and net warming of the permafrost at the bottom.

Even though a two phase system is more efficient in terms of heat removed per pound of refrigerant circulated. because of the small amount of heat required to be removed and the slow speed of the process, efficiency is not an essential requirement. Accordingly, it is deemed critical to maintain the refrigerant in-the liquid state throughout the system and to impose sufficient pressure upon the system to prevent vaporization throughout the operating temperature range. While the actual pressure required to suppress vaporization depends upon the particular refrigerant selected and the operating temperatures, in most instances vaporization can be suppressed at a pressure of between about psig and 200 psig, measured at the surface. Preferably, the

pressure is maintained at least about l0 psig above the Nominal Element Diameter, Weight. Number Inches lbs/ft 2U 30 157.6 24 94 l 3% 68 34 9% 53.5 38 4V2 I Casing 24 and the refrigerant chamber extends from the surface to a depth of 300 feet. Refrigerant tubes 54 and 56 are 2-inch ID pipe provided with 0.312 inches of insulation. The cross-sectional flow area of each refrigerant tube is 0.02 l 82 square feet and the net crosssectional flow area of refrigerant chamber 52 is 0.9289 square feet, which provides a flow area ratio of about 2L3 to l.

The refrigerant system is charged with normal butane at a pressure of 50 psig. Temperature control valve 68 is opened and circulation of the refrigerant established. At a heat flux of 20 BTU/hr per lineal foot of well, the refrigerant circulation is [.76 cubic feet per minute. At this circulation rate, the velocity in refrigerant tubes 54 and 56 is 0.67 feet per second. and the velocity in refrigerant chamber 52 is 0.03 feet per second. The calculated pressure drop through the complete refrigerant system is 13.7 psi, of which 11.4 psi is in refrigerant tubes 54 and 56.

At a mean air temperature of 20 F., the temperature of the refrigerant exiting the exchanger is -l6 F. and the temperature of the refrigerant discharged into the bottom of the refrigeration chamber is about 5.9 F. The temperature rise of the refrigerant in the refrigeration chamber is about 2.7 Fr, thus the temperature of the refrigerant exiting refrigeration chamber 52 and being recirculated to air cooler 64 is about l3.2 F. When operating under the mean ambient temperature conditions illustrated in FIG. 3, the permafrost is cooled substantially as illustrated in FIG. 4, even though hot oil at a temperature of about F. is being produced through tubing 38. It is apparent from FIG. 4 that over an annual cycle of 5 years, that not only is thawing of the permafrost prevented, the permafrost is in fact cooled to lower than normal temperatures.

Various embodiments and modifications of this invention have been described in the foregoing specification, and further modifications will be apparent to those skilled in the art. Such modifications are included within the scope of this invention as defined by the following claims.

Having now described our invention, we claim:

I. A refrigerated well for conducting hot fluids through a permafrost zone. which comprises:

a first casing extending a substantial distance into said permafrost zone and cemented therein;

a second casing placed concentrically within said first casing, said second casing extending below said first casing and said second casing being cemented below the bottom of said first casing;

sealing means at the top and the bottom of said first casing for providing a fluid tight closure between said first and second casings so as to form a closed annular refrigeration chamber;

a production tubing within said second casing to convey fluids from an underlying producing strata to the surface or vice versa, said tubing being insulated through said permafrost zone;

at least one insulated refrigerant tube in said annular refrigeration chamber extending from the surface to the bottom of said chamber;

a heat exchanger at the surface for conducting a refrigerant in heat exchange relationship with ambient air;

conduit means for connecting said annular refrigeration chamber to said heat exchanger and for connecting said heat exchanger to the upper end of each of said refrigerant tubes to provide a closed refrigeration circuit;

valve means in said conduit means for stopping the circulation of regrigerant through said refrigeration circuit;

a reservoir of liquid refrigerant connected to said refrigeration circuit; and

means for introducing gas under pressure into said reservoir and maintaining said gas at a pressure sufficient to prevent vaporization of said liquid refrigerant in said refrigeration circuit. 2. The apparatus defined in claim I wherein the ratio of effective cross-sectional flow area of said annular refrigeration chamber to that of the refrigeration tubes is from about lzl to about 41l.

3. The apparatus defined in claim 1 wherein the ratio of effective cross-sectional flow area of said annular refrigeration chamber to that of the refrigeration tubes is from about 30:l to about l0:l.

4. The apparatus defined in claim 1 including temperature responsive means to close said valve means upon the ambient air temperature rising above a predetermined value.

5. The apparatus defined in claim 1 including temperature responsive means to close said valve means upon the difference between the ambient air temperature and the permafrost temperature reaching a predetermined value.

6. The apparatus defined in claim l wherein said first casing extends to the bottom of said permafrost zone.

7. A refrigerated well for conducting hot fluids through a permafrost zone. which comprises:

a surface conductor cemented in the permafrost;

a first casing placed concentrically within said surface conductor and extending a substantial distance into said permafrost zone. said first casing being cemented to the surface;

a second casing placed concentrically within said first casing. said second casing extending below said first casing and said second casing being cemented below the bottom of said first casing;

sealing means at the top and the bottom of said first casing for providing a fluid tight closure between said first and said second casing so as to form a closed annular refrigeration chamber;

a production tubing within said second casing to convey fluids from an underlying producing strata to the surface or vice versa. said tubing being insulated through said permafrost zone;

at least one insulated refrigerant tube in said annular refrigeration chamber extending from the surface to the bottom of said chamber, the number and size of said refrigerant tubes being such that the ratio of effective cross-sectional flow area of said annular refrigeration chamber to that of the refrigeration tubes is from about 30:1 to :1;

a heat exchanger at the surface for conducting a refrigerant in heat exchange relationship with ambient air;

conduit means for connecting said annular refrigeration chamber to said heat exchanger and for connecting said heat exchanger to the upper end of each of said refrigerant tubes to provide a closed refrigeration circuit;

valve means in said conduit means for stopping the circulation of refrigerant through said refrigeration circuit;

a reservoir of liquid refrigerant connected to said refrigeration circuit; and

means for introducing gas under pressure into said reservoir and maintaining said gas at a pressure sufficient to prevent vaporization of said liquid refrigerant in said refrigeration circuit.

8. The apparatus defined in claim 7 including temperature responsive means to close said valve means upon the ambient air temperature rising above a prede termined value.

9. The apparatus defined in claim 6 including temperature responsive means to close said valve means upon the difference between the ambient air temperature and permafrost temperature reaching a predetermined value.

l0. The apparatus defined in claim 7 including one or more intermediate casings substantially concentrically placed within said second casing between said second casing and said production tubing and extending below said second casing. said intermediate casing being cemented below the bottom of said next outer casing.

H. A method for conducting hot fluids through a well traversing a permafrost zone without melting the permafrost surrounding the well. which comprises:

flowing the hot fluid through an insulated first tubular member within said well; during the period that the ambient air temperature is below the temperature of the permafrost. circulating a cold liquid refrigerant by natural convection flow downwardly through an insulated refrigerant tube at relatively high velocity and upwardly through an annular refrigeration chamber surrounding said insulated first tubular member to remove heat transferred from said hot fluid and to subcool the permafrost surrounding the well;

cooling the refrigerant exiting said refrigeration chamber by heat exchange with the ambient air;

maintaining sufficient pressure on the refrigerant system to maintain the refrigerant in the liquid phase; and

discontinuing the circulation of refrigerant when the ambient air temperature exceeds a predetermined temperature of not greater than the temperature of the permafrost.

13. The method defined in claim 11 wherein the velocity of said refrigerant in said refrigerant tubes is about 0.2 to 1 foot per second. and the velocity of said refrigerant in said refrigeration chamber is about 0.01 to 0.05 foot per second.

14. A method for conducting hot fluids through a well traversing a permafrost zone without melting the permafrost surrounding the well. which comprises:

flowing said hot fluid substantially continuously through an insulated first tubular member within said well;

during periods that the ambient air temperature is below the temperature of the permafrost. circulating a cold liquid refrigerant selected from the group consisting of low molecular weight hydrocarbons, halogenated hydrocarbons. and mixtures thereof downwardly through an insulated refrigerant tube at a velocity of about 0.2 to l foot per second and upwardly through an annular refrigeration chamber surrounding said insulated first tubular member at a velocity of 0.01 to 0.05 foot per second to remove heat transferred from said hot fluid and to subcool the permafrost surrounding the well;

throughout said system; and

discontinuing the circulation of refrigerant when the ambient air temperature exceeds a predetermined temperature not greater than the temperature of the permafrost.